1. Field of the Invention
The present invention relates to thermosetting powder coating compositions and curing agent compositions, and more particularly to thermosetting powder coating compositions excellent in storage stability and capable of providing coating films with excellent appearance and also to curing agent compositions for use in the thermosetting powder coating compositions.
2. Description of Related Art
Powder coatings which do not release organic solvents to an atmosphere have been widely noted as eco-friendly coatings.
Currently, the use of thermosetting powder coatings is popular in the powder coating field for their ability to provide coating films excellent in performances and physical properties. For their application to automotive bodies, the improvements in smoothness and appearance of resulting films have been demanded. One technique used to improve the film smoothness is the thick application of powder coatings. Another technique is the use of powder coatings prepared from the materials having low melt viscosity. While effective in achieving some improvements in smoothness and appearance of resulting films on automobile horizontal portions such as a hood and the like, these techniques have suffered from the problem. That is, the powder coatings applied onto automobile vertical portions such as doors and the like, when subsequently heated and melted, result in coating films which show a marked reduction in smoothness and appearance as a result of the occurrence of sagging.
The incorporation of fine crosslinked resin particles, called microgels, to prevent the occurrence of sagging is known in the solvent-borne coating field. For example, Japanese Patent Laying-Open Nos. Sho 49-97026 (1974) and Sho 60-250068 (1985) disclose techniques which contemplate to reduce sagging and accordingly improve film appearance by adding microgels to top coatings.
In the powder coating field, the use of powder coatings containing fine crosslinked resin particles is disclosed, for example, in Japanese Patent Laying-Open Nos. Sho 62-97026 (1987). However, such powder coatings are prepared by mixing dry fine particles of crosslinked resin under a solvent-free condition. During the mixing, the crosslinked resin particles are often caused to locally form agglomerates. This disturbs uniform distribution thereof to result in the unsatisfactory smoothness and appearance of obtaining coating films. In the case where a melt mixing process is added to precede the dry mixing process, a majority of fine particles of crosslinked resin are caused to deposit on surfaces of resulting powder coating particles. When such powder coating particles are applied and subsequently heated, the interaction is caused to occur between the fine crosslinked resin particles. This has led often to the insufficient smoothness and appearance of resulting coating films.
Powder coatings, when heated to melt, result in coating films. However, such coating films show the insufficient smoothness compared to those obtained from solvent-borne coatings. In order to improve the smoothness of resulting coating films, the melt viscosities of components contained in the powder coatings, i.e., the melt viscosity of raw material must be reduced to improve the fluidity thereof during melt.
One approach to lower the melt viscosity of raw material may be the use of low-melting point or low-molecular-weight substances for the raw material. While effective in improving the smoothness of resulting films, their use has also led to the reduction in storage properties, such as resistance to blocking and solid reaction. The difficulty has thus been to reconcile the storage stability of the coating composition and the smoothness of resulting film.
In Japanese Patent Laying-Open No. Hei 9-100414 (1997), a method is disclosed which produces thermosetting resin particles having a narrow particle size distribution by utilizing a wet process. Since the thermosetting resin particles are produced in an aqueous medium, their use has led to the improvement in one of storage properties, i.e., resistance to solid reaction. It has been unsatisfactory, however, to reconcile the blocking resistance of the powder coating composition and the smoothness of the obtaining film.
Among thermosetting powder coatings, acrylic-based coatings, when formed into films, provide excellent performances and physical properties. The resulting films however suffer from a problem of poor appearance, called a hazing phenomenon. This phenomenon is known to often occur when a large amount of polybasic acid is used as a curing agent. If the reduced amount of polybasic acid is used to suppress the occurrence of such a hazing phenomenon, the performances and physical properties of resulting films then become insufficient. Also, there exists no polybasic acid which can act to improve performances and physical properties of resulting films while controlling the occurrence of the hazing phenomenon, so far as we know.